Militias in Libya Advance on ISIS Stronghold of Surt With Separate Agendas

CAIRO — Fighters aligned with Libya’s United Nations-backed unity government are advancing along the Mediterranean coast toward the Islamic State stronghold of Surt, signaling the first major assault on territory that, since last year, has become the terrorist group’s largest base outside Iraq and Syria.

Two separate militia forces have fought their way toward the city in recent days, attacking from both the east and the west, in apparently uncoordinated attacks that have reduced the length of Libyan coastline controlled by the Islamic State to 100 miles from about 150 miles. On Wednesday, one of the militias claimed to have seized control of Surt’s power plant, 20 miles west of the city.

Those victories occurred in sparsely populated areas, and it was unclear whether the militias had either the strength or the will to push into Surt, which is thought to be heavily fortified and also harbor several thousand foreign fighters. But the advance did signal a new setback for the Islamic State, also known as ISIS or ISIL, at a time when it is already under concerted attack in Falluja, Iraq, and in parts of Syria.

Analysts and diplomats warn that while the offensive addresses the West’s biggest concern in Libya, it also risks destabilizing the fragile peace effort there by fostering violent competition between rival groups.

“Only a year ago, these two groups were battling for control of the so-called oil crescent, and lobbing rockets and shells at one another,” said Frederic Wehrey, a Libya specialist at the Carnegie Endowment for International Peace, who visited that country recently. “Now they are converging on a common enemy, but the great fear is what comes next.”

Over the past year, Surt, the hometown of the ousted Libyan dictator Col. Muammar el-Qaddafi, has become a preoccupation for Western countries worried that it could become a refuge for militants fleeing Iraq and Syria. Islamic State fighters have presided over a brutal rule in the city, with public executions and floggings, as well as shortages of food and medicines.

Since late 2015, small groups of American, British and French special operations forces have quietly deployed across Libya, making contact with friendly Libyan militias in an effort to gather intelligence on the Islamic State. In February, the Pentagon presented the White House with a potential plan for extensive airstrikes against the militant group’s camps, command centers and munitions depots in Libya.

But President Obama has stayed his hand, limiting overt American action to sporadic strikes against a handful of Islamic State targets in an effort to allow a United Nations-led peace process to take root in Libya. That endeavor, however, has faltered badly as the unity government, which arrived in the capital, Tripoli, in March, has failed to gain broad political acceptance.

Now, with the sudden move against the Islamic State, military action on the ground is moving faster than the country’s tangled politics.

The power plant where fighting raged on Wednesday is a significant prize because its loss to the Islamic State last June was seen as a significant step in the group’s domination of the Surt region. The assault was instigated by militias from Misurata, a powerful trading city further west along the coast, in response to Islamic State attacks. Dozens of Misuratan fighters have died in recent weeks, according to Libyan media reports.

Hamza Ahmed Abusnaina, a senior Misuratan commander, said in a telephone interview on Wednesday that the attackers had captured both the Surt power plant and an area south of the city called Wadi Jaref. Mr. Abusnaina’s claim could not be independently confirmed, but it was echoed by Twitter accounts associated with the Misuratan militias.

On the eastern side of the city, the attack is led by Ibrahim Jathran, a young militia commander who controls a stretch of coast known as the oil crescent, where most of Libya’s oil terminals are. In recent days, his group seized the coastal town of Bin Jawad and claimed on Tuesday to have moved on nearby Nawfaliyah.

That would bring his group, known as the Petroleum Facilities Guard, within 80 miles of Surt.

It is unclear whether foreign forces are playing a direct role in the offensive. In The Times of London last week, a report from Misurata cited a local commander who said that British special forces soldiers had fired a missile to destroy an Islamic State truck packed with explosives during a battle in early May. British defense officials did not comment on the report.

In April, a Pentagon spokesman said that the small group of United States Special Operations forces deployed to Libya — about two dozen troops operating near Misurata and Benghazi — were principally involved in intelligence gathering and reconnaissance.

As the two-pronged assault on Islamic State territory unfolded, several analysts pointed to the role of the unity’s government’s new defense minister, Almahdi Al-Barghathi, who has been trying to bring rival militant factions under a central command that could become a national army. But such efforts are being frustrated by the tribal and personal rivalries that have fueled chaos in Libya since the fall of Colonel Qaddafi in 2011.

“These forces lack crucial capabilities,” said Mr. Wehrey, of the two groups. “It’s one thing to push back I.S. in the surrounding villages and towns but quite another to liberate Surt.”

The coastal city is thought to be home to a majority of the Islamic State fighters in Libya, estimated to number between 3,000 and 6,500. Western officials say that any group attacking Surt would likely face suicide bombers, roadside bombs and the resistance of fighters with few avenues of escape. It is estimated that two-thirds of the city’s population has already fled.

More broadly, there is a danger of deepening divisions between east and west in Libya. One Western official who recently visited the country said the political mood in Libya had become increasingly confrontational during recent months as the United Nations, acting under pressure from the United States and its allies, has struggled to win acceptance for the unity government.

The assault on Surt could further isolate Gen. Khalifa Hifter, a powerful militia commander in the eastern city of Benghazi, who has determinedly resisted all entreaties to join the unity government. Only weeks ago, he boasted that he would be the one to rout the Islamic State from Surt.

In a sign of those divisions, the eastern branch of the country’s central bank this week announced that it had printed 4 billion Libyan dinars through a company in Russia, drawing a furious reaction from the main central bank in Tripoli.

An article on June 2 about a military advancement toward the Islamic State stronghold of Surt, Libya, by fighters aligned with the country’s United Nations-backed government misstated part of a comment by Frederic Wehrey, a Libya specialist at the Carnegie Endowment for International Peace who visited the country recently. Referring to rival militias in Libya, he said, “Only a year ago, these two groups were battling for control of the so-called oil crescent, and lobbing” — not “lobbying” — “rockets and shells at one another.”

A version of this article appears in print on , on Page A9 of the New York edition with the headline: Militias in Libya Advance on ISIS Stronghold, Separate Agendas in Tow. Order Reprints | Today’s Paper | Subscribe